Gas water heater draft hoods provide additional air for the combustion process gases, to be properly pulled out from the burner chamber (base of the water heater), into the water heater vent pipe and the chimney.
Mycelium is a block added in Minecraft the same time as the introduction of the Mushroom Biomes in Minecraft 1.9pre1. Mycelium can only be found and only can be found in Mushroom Biomes. Mycelium resembles dirt in that it can be broken with a shovel and it without a 'Silk Touch' enchanted tool it only drops dirt when broken. Mycelium also lets off a small particle effect the same as Bedrock.
The living body of the fungus is a mycelium made out of a web of tiny filaments called hyphae. The mycelium is usually hidden in the soil, in wood, or another food source. A mycelium may fill a single ant, or cover many acres. The branching hyphae can add over a half mile (1 km) of total length to the mycelium each day.